The present invention relates to a twin-wire former for a paper making machine. Each wire is an endless loop screen onto which pulp suspension from a headbox is directed. A twin-wire former has two wires and the pulp suspension is sandwiched between the wires.
Numerous twin wire formers for paper machines are known, and they include many modifications. Reference may be had to a number of publications:
(1) Federal Republic of Germany 39 10 892 A 1 describes a twin-wire former which contains several known features. The wire support frame is not shown in the reference. But, it is present, as a person skilled in the art knows. The forming roll which is located at the beginning of the forming or wire section has a perforated roll jacket. The web forming zone has a first curved section. A plurality of transverse ledges, which extend transversely to the direction of travel of the lower wire, are arranged within the lower wire loop. Water removal elements are also present within the upper wire loop. The web forming zone extends at a slight angle to the horizontal.
(2) Federal Republic of Germany 39 27 597 A 1 describes several variants of twin-wire formers. In the variant shown in FIG. 1, the web forming zone also has the shape of a circular arc with a very large radius of curvature. The initial region of the web forming zone is also slightly inclined to the horizontal. In the variants shown in FIGS. 2 and 3, the web forming zone rises substantially vertically.
(3) U.S. Pat. No. 3,846,232 describes a twin-wire former having a head box, and the outlet channel of the headbox rises at an angle of about 30.degree. to the horizontal. The jet of pulp is sandwiched between two forming wires which wrap around a forming roll, and the wires leave the forming roll, together with the fiber web between them, at an angle of about 30.degree. to the vertical. The web forming zone thus rises very rapidly from that point.
(4) The "Speed Former HS" article from the 1988 annual meeting technical section, CPPA, describes a twin-wire former in which the jet of pulp, similar to Reference (3) above, is injected obliquely from below into the entrance slot between two wires and in which the two wires with the web present between them leave the forming roll at a rather steep angle. The reference shows a part of the wire frame which comprises horizontal beams and vertical supports.
A twin-wire former must satisfy numerous requirements. It must form a good web or sheet, i.e. the sheet should be of perfect quality with regard, for instance, to fiber distribution, cloudiness, as well as having uniform basis weight over the width. These requirements relate to the paper which is to be produced. Other requirements concern the construction of the machine. Thus, the twin-wire former should be as simple as possible in construction in order to keep its manufacturing costs low. It is further important that the twin-wire former be developed favorably for performing its functions. In this connection, it is important, for instance, that the large amount of water which emerges from the web be led away dependably and reliably. The wires must be able to be easily and well cleaned. They should be subject to the smallest possible amount of wear so that they are capable of removing water even after prolonged use.
The comfort of operation of the twin-wire former is particularly important. The twin-wire former should be developed so that the paper making machine crew can easily and rapidly take all necessary steps and so that the fewest number of people is required for machine operation. Thus, the water removal elements should be easily accessible, so that they can also be easily replaced or adjusted. This is specifically not true of known twin-wire formers. The head box is continuously subject to dirtying. Because it is an important part of the paper making machine, the headbox must be cleaned regularly. In this connection, it is desirable to arrange the headbox so that it is less subject to dirt and so that it furthermore is easily accessible for cleaning and other servicing.
Another very important requirement is that the dimensions of the twin-wire former, including the headbox, be kept as small as possible. In this connection, furthermore, the possibility of expansion must be borne in mind.
Known twin-wire formers have in each case satisfied one or more of these requirements. In this connection, however, it has often been found that it is difficult to satisfy all of the above objectives. In particular, the requirement as to the space taken up has not been sufficiently handled.